1. Field of the Invention
The invention relates to a method for the electrodeposition of ruthenium and in particular to a method for the electrodeposition of relatively thick layers of ruthenium.
2. Description of the Prior Art
Increasingly, attention has been focused on the use of ruthenium in electrical contact applications, such as in reed switches. It is priorly known that a reed switch contact may be formed on a nickel-iron alloy contact support or reed by depositing a gold layer on the nickel-iron alloy surface and thereafter depositing a layer of ruthenium. Examples of such reed switch contacts may be found in U.S. Pat. Nos. 3,663,777 issued May 16, 1972; 3,889,098 issued June 10, 1975; and 3,916,132 issued Oct. 28, 1975 all to A. Steinmetz et al. Typically, such prior art ruthenium contacts are deposited by means of sputtering techniques. The ruthenium layers so formed are relatively thin.
Another technique of forming relatively thin ruthenium layers is the electrodeposition of a ruthenium coating by employing an aqueous electrolyte solution of ruthenium in conjunction with continuous direct current densities.
It is desirable in certain instances to provide relatively thick, e.g., 20 to 45 microinches (0.5 to 1.125 micrometers), coatings. One problem with such coatings obtained by conventional techniques is that the ruthenium coating is characterized by severe internal stress resulting in the formation of cracks in the coating. This problem has been found to become more significant where a thick ruthenium layer is electrodeposited over a remanently magnetic material of the type generally known as "remendur." One example of a remendur composition is taught in U.S. Pat. No. 3,364,449 issued Jan. 16, 1968 to H. L. B. Gould et al.
One solution to the general problem of obtaining thick stress-free electrodeposits of ruthenium is disclosed in U.S. Pat. No. 3,630,856 issued Dec. 28, 1971 to A. Meyer. According to that patent, a stress-free ruthenium deposit may be obtained by utilizing an indium, gallium, or thallium addition to the electrolyte plating solution.
In Gold Plating Techniques, F. H. Reed et al, Electrochemical Publications Limited, Scotland, 1974, at page 65 it is reported that a technique known as pulsed current plating may be employed to produce a less nodular, finer grain deposit of gold than can be obtained using direct current electrodeposition. According to this technique, the plating current source produced a pulsed current output, i.e., a current is generated for a first time period and is absent during a second time period, the first and second time periods reoccur cyclically.